Synthesis of bowl-shaped polycyclic aromatic hydrocarbons via palladium-catalyzed intramolecular arylation reactions

Article abstract of DOI:10.1021/ol0605676

Treatment of the benzannulated enediyne 11 with potassium tert-butoxide in refluxing toluene for 12 h produced 15 via a cascade sequence of cyclization reactions. Two subsequent palladium-catalyzed intramolecular arylation reactions then afforded the bowl

Full text of DOI:10.1021/ol0605676

ORGANIC
LETTERS
2006
Vol. 8, No. 11
2313-2316
Synthesis of Bowl-Shaped Polycyclic
Aromatic Hydrocarbons via
Palladium-Catalyzed Intramolecular
Arylation Reactions
Daehwan Kim, Jeffrey L. Petersen,† and Kung K. Wang*
C. Eugene Bennett Department of Chemistry, West Virginia UniVersity,
Morgantown, West Virginia 26506-6045
kung.wang@mail.wVu.edu
Received March 8, 2006
ABSTRACT
Treatment of the benzannulated enediyne 11 with potassium tert-butoxide in refluxing toluene for 12 h produced 15 via a cascade sequence
of cyclization reactions. Two subsequent palladium-catalyzed intramolecular arylation reactions then afforded the bowl-shaped polycyclic
aromatic hydrocarbon 16. The X-ray structures of 16 and two closely related molecules show the presence of significant curvatures.
Bowl-shaped polycyclic aromatic hydrocarbons (bucky-
bowls) have received considerable attention in recent years.1
This is due in part to the possibility of using these curved
hydrocarbons as building blocks for the construction of
fullerenes.2 In addition, the chemical reactivities of the more
accessible concave side of the interior carbon atoms of
buckybowls could mimic the endohedral chemistry of
fullerenes.3 Furthermore, buckybowls are of interest them-
selves for a variety of reasons. They provide a platform for
the study of the effect of pyramidalization of sp2-hybridized
carbons on aromaticity.4 On exposure to lithium metal,
corannulene (1), a C20H10 buckybowl, was found to form a
fascinating tetraanion.5 Reports of complexation of bucky-
bowls with transition metals have also begun to emerge.6
The first synthesis of corannulene was achieved via a
multistep synthetic sequence.7 The use of flash vacuum
pyrolysis to connect distantly separated carbon atoms in
(4) (a) Ferrer, S. M.; Molina, J. M. J. Comput. Chem. 1999, 20, 1412-
1421. (b) Steiner, E.; Fowler, P. W.; Jenneskens, L. W. Angew. Chem., Int.
Ed. 2001, 40, 362-366. (c) Poater, J.; Fradera, X.; Duran, M.; Sola`, M.
Chem. Eur. J. 2003, 9, 1113-1122.
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H. B., Jr.; Lawton, R. G. J. Am. Chem. Soc. 1967, 89, 3056-3058. (b)
Ayalon, A.; Rabinovitz, M.; Cheng, P.-C.; Scott, L. T. Angew. Chem., Int.
Ed. Engl. 1992, 31, 1636-1637. (c) Ayalon, A.; Sygula, A.; Cheng, P.-C.;
Rabinovitz, M.; Rabideau, P. W.; Scott L. T. Science 1994, 265, 1065-
1067. (d) Aprahamian, I.; Preda, D. V.; Bancu, M.; Belanger, A. P.;
Sheradsky, T.; Scott, L. T.; Rabinovitz, M. J. Org. Chem. 2006, 71, 290-
298.
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Am. Chem. Soc. 1997, 119, 4781-4782. (b) Vecchi, P. A.; Alvarez, C. M.;
Ellern, A.; Angelici, R. J.; Sygula, A.; Sygula, R.; Rabideau, P. W. Angew.
Chem., Int. Ed. 2004, 43, 4497-4500. (c) Petrukhina, M. A.; Andreini, K.
W. Organometallics 2005, 24, 1394-1397. (d) Stoddart, M. W.; Brownie,
J. H.; Baird, M. C.; Schmider, H. L. J. Organomet. Chem. 2005, 690, 3440-
3450. (e) Petrukhina, M. A.; Scott, L. T. J. Chem. Soc., Dalton Trans. 2005,
18, 2969-2975. (f) Vecchi, P. A.; Alvarez, C. M.; Ellern, A.; Angelici, R.
J.; Sygula, A.; Sygula, R.; Rabideau, P. W. Organometallics 2005, 24,
4543-4552.
† To whom correspondence concerning the X-ray structures should be
addressed. Phone: (304) 293-3435, ext 6423. Fax: (304) 293-4904.
E-mail: jeffrey.petersen@mail.wvu.edu.
(1) For reviews, see: (a) Rabideau, P. W.; Sygula, A. Acc. Chem. Res.
1996, 29, 235-242. (b) Scott, L. T. Pure Appl. Chem. 1996, 68, 291-300.
(c) Mehta, G.; Rao, H. S. P. Tetrahedron 1998, 54, 13325-13370. (d) Scott,
L. T.; Bronstein, H. E.; Preda, D. V.; Ansems, R. B. M.; Bratcher, M. S.;
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10.1021/ol0605676 CCC: $33.50
© 2006 American Chemical Society
Published on Web 05/04/2006

11H-benzo[b]fluorenes for the subsequent palladium-cata-
lyzed intramolecular arylation reactions leading to bucky-
bowls.
The requisite (2,6-dibromophenyl)ethyne (5) was prepared
by the Sonogashira reaction between 1,3-dibromo-2-iodo-
benzene and (trimethylsilyl)ethyne followed by desilylation
as reported previously.15 A second Sonogashira reaction
between 5 and [(2-iodophenyl)ethynyl]trimethylsilane (6)16
then led to 7, which was readily desilyated to afford the
benzannulated enediyne 8 (Scheme 1). Condensation between
Scheme 1
planar polycyclic aromatic precursors provided a more direct
route to a host of buckybowls,1 including corannulene,8
diindeno[1,2,3,4-defg;1′,2′,3′,4′-mnop]chrysene (2),9 semi-
buckminsterfullerene 3,10 and circumtrindene (4),11 culminat-
ing in the successful synthesis of C60 from a C60H27Cl3
molecule.12 Several efficient nonpyrolytic methods for
buckybowls were also reported,13 including the titanium-,
vanadium-, and nickel-mediated intramolecular reductive
coupling of benzylic and benzylidene bromides,13a-d,j the
intramolecular carbenoid coupling of dibromomethyl
groups,13g-i and the palladium-catalyzed intramolecular aryl-
ation of aryl halides.13e,f We recently reported an efficient
route to 5-phenyl-11H-benzo[b]fluorenes involving conden-
sation between benzannulated enediynes and aryl ketones
to produce benzannulated enediynyl propargylic alcohols
followed by reduction and a sequence of cascade cyclization
reactions.14 We have successfully adopted this synthetic
method to prepare the corresponding 5-(2,6-dibromophenyl)-
(7) Barth, W. E.; Lawton, R. G. J. Am. Chem. Soc. 1966, 88, 380-381.
(b) Barth, W. E.; Lawton, R. G. J. Am. Chem. Soc. 1971, 93, 1730-1745.
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Am. Chem. Soc. 1991, 113, 7082-7084. (b) Borchardt, A.; Fuchicello, A.;
Kilway, K. V.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1992,
114, 1921-1923. (c) Zimmermann, G.; Nuechter, U.; Hagen, S.; Nuechter,
M. Tetrahedron Lett. 1994, 35, 4747-4750. (d) Liu, C. Z.; Rabideau, P.
W. Tetrahedron Lett. 1996, 37, 3437-3440. (e) Scott, L. T.; Cheng, P.-C.;
Hashemi, M. M.; Bratcher, M. S.; Meyer, D. T.; Warren, H. B. J. Am.
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N.; Scott, L. T. J. Am. Chem. Soc. 2002, 124, 8870-8875.
8 and pivalophenone (9) then produced enediynyl propargylic
alcohol 10. Treatment of 10 with triethylsilane in the presence
of trifluoroacetic acid then afforded 11. On exposure to
potassium tert-butoxide in refluxing toluene for 12 h, 11 was
converted to 5-(2,6-dibromophenyl)-10-(1,1-dimethylethyl)-
11H-benzo[b]fluorene (15) in a single operation. The trans-
formation from 11 to 15 presumably proceeded through an
initial 1,3-protropic rearrangement to form the corresponding
benzannulated enyne-allene 12. A Schmittel cyclization
reaction17 to generate biradical 13 for an intramolecular
radical-radical coupling to afford, in situ, 14 followed by a
(10) (a) Rabideau, P. W.; Abdourazak, A. H.; Folsom, H. E.; Marcinow,
Z.; Sygula, A.; Sygula, R. J. Am. Chem. Soc. 1994, 116, 7891-7892. (b)
Hagen, S.; Bratcher, M. S.; Erickson, M. S.; Zimmermann, G.; Scott, L. T.
Angew. Chem., Int. Ed. Engl. 1997, 36, 406-408. (c) Mehta, G.; Panda,
G. Chem. Commun. 1997, 2081-2082.
(11) Ansems, R. B. M.; Scott, L. T. J. Am. Chem. Soc. 2000, 122, 2719-
2724.
(12) Scott, L. T.; Boorum, M. M.; McMahon, B. J.; Hagen, S.; Mack,
J.; Blank, J.; Wegner, H.; de Meijere, A. Science 2002, 295, 1500-1503.
(13) (a) Seiders, T. J.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc.
1996, 118, 2754-2755. (b) Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc.
1998, 120, 12666-12667. (c) Sygula, A.; Rabideau, P. W. J. Am. Chem.
Soc. 1999, 121, 7800-7803. (d) Seiders, T. J.; Elliott, E. L.; Grube, G. H.;
Siegel, J. S. J. Am. Chem. Soc. 1999, 121, 7804-7813. (e) Reisch, H. A.;
Bratcher, M. S.; Scott, L. T. Org. Lett. 2000, 2, 1427-1430. (f) Wang, L.;
Shevlin, P. B. Org. Lett. 2000, 2, 3703-3705. (g) Sygula, A.; Marcinow,
Z.; Fronczek, F. R.; Guzei, I.; Rabideau, P. W. Chem. Commun. 2000,
2439-2440. (h) Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc. 2000, 122,
6323-6324. (i) Sygula, A.; Xu, G.; Marcinow, Z.; Rabideau, P. W.
Tetrahedron 2001, 57, 3637-3644. (j) Sygula, A.; Karlen, S. D.; Sygula,
R.; Rabideau, P. W. Org. Lett. 2002, 4, 3135-3137.
(14) Li, H.; Zhang, H.-R.; Petersen, J. L.; Wang, K. K. J. Org. Chem.
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2004, 60, 4451-4458.
2314
Org. Lett., Vol. 8, No. 11, 2006

second prototropic rearrangement to regain aromaticity then
furnished 15 as proposed previously.14
It was gratifying to observe that treatment of 15 with 10
mol % of Pd(PPh3)2Br2 in the presence of DBU under the
conditions reported by Scott et al.13e produced buckybowl
16 (Scheme 2). The structure of 16 was confirmed by X-ray
to give 17. Analysis of the crude reaction mixture by GC/
MS before purification by silica gel chromatography also
suggests the presence of a small amount of 18.
The X-ray structure of 16 indicates the presence of a
significant curvature. Compared to 2, the structure of 16
appears to be less strained with one less ring due to the
absence of three sp2-carbons in the southeastern corner. The
pyramidalization angle, defined as Θσπ - 90 using the
π-orbital axis vector analysis (POAV1),19 of the central
ethylene carbon atoms of 2 was reported to be 9.0°, whereas
the pyramidalization angle of the four carbon atoms
attached to the central double bond was determined to be
6.7° (Figure 2).9b The POAV1 angles of these carbon atoms
Scheme 2
Figure 2. POAV1 pyramidalization angles (Θσπ - 90) of 2 and
16.
structure analysis (Figure 1). Similar to what was observed
are clearly larger than those of the corresponding carbon
atoms of 16 based on the geometry obtained from the X-ray
analysis.
The 1H NMR signal of the diastereotopic methylene
hydrogens of 16 occurs as a singlet at δ 4.75 ppm. Barring
accidental isochrony, this observation suggests a rapid bowl-
to-bowl inversion on the NMR time scale at room temper-
ature, reminiscent of what was observed previously in
substituted corannulene derivatives.13a,20
Similarly, by using aryl ketones 19a14 and 19b14 for
condensation with the lithium acetylide of 8, benzannulated
enediynyl propargylic alcohols 20a and 20b were obtained,
respectively. Reduction with triethylsilane in the presence
of trifluoroacetic acid then gave benzannulated enediynes
21a and 21b. Treatment of 21a and 21b with potassium tert-
butoxide in refluxing toluene for 12 h then furnished 22a
and 22b, respectively. The palladium-catalyzed intramolecu-
lar arylation reactions of 22a produced buckybowl 23a and
the monocyclized hydrocarbon 24a. The X-ray structure of
23a also indicates the presence of a significant curvature.
Similarly, buckybowl 23b along with the monocyclized
hydrocarbon 24b were likewise produced from 22b. The
structure of 23b was also confirmed by X-ray structure
analysis (Figure 3). The presence of an additional five-
Figure 1. X-ray crystal structure of 16.
previously,13e reductive debromination either on one side or
on both sides also occurred, giving rise to the monoclosed
hydrocarbon 17 as a minor product and a small amount of
18, respectively. The structure of 17 was tentatively assigned
based on the assumption that the first cyclopalladation
reaction of 15 occurred preferentially with the naphthyl
moiety of 15 producing a five-membered ring as reported
previously.18 The resulting brominated fluoranthene inter-
mediate could undergo either a second cyclopalladation
reaction to produce 16 or a reductive debromination reaction
(19) (a) Haddon, R. C.; Scott, L. T. Pure Appl. Chem. 1986, 58, 137-
142. (b) Haddon, R. C. J. Am. Chem. Soc. 1987, 109, 1676-1685. (c)
Haddon, R. C. Science 1993, 261, 1545-1550.
(20) (a) Scott, L. T.; Hashemi, M. M.; Bratcher, M. S. J. Am. Chem.
Soc. 1992, 114, 1920-1921. (b) Sygula, A.; Rabideau, P. W. THEOCHEM
1995, 333, 215-226. (c) Biedermann, P. U.; Pogodin, S.; Agranat, I. J.
Org. Chem. 1999, 64, 3655-3662. (d) Marcinow, Z.; Sygula, A.; Ellern,
A.; Rabideau, P. W. Org. Lett. 2001, 3, 3527-3529. (e) Seiders, T. J.;
Baldridge, K. K.; Grube, G. H.; Siegel, J. S. J. Am. Chem. Soc. 2001, 123,
517-525.
(17) (a) Schmittel, M.; Strittmatter, M.; Vollmann, K.; Kiau, S. Tetra-
hedron Lett. 1996, 37, 999-1002. (b) Schmittel, M.; Strittmatter, M.; Kiau,
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(18) Rice, J. E.; Cai, Z.-W. J. Org. Chem. 1993, 58, 1415-1424.
Org. Lett., Vol. 8, No. 11, 2006
2315

atoms having a POAV1 angle of 11.64°. In addition, one of
the four carbon atoms attached to the central double bond
of 23b has a POAV1 angle of 9.2°, which is also larger than
those of the corresponding carbon atoms of 2.
It was reported that, unlike corannulene, dihydrocyclo-
pentacorannulene (25) with an additional five-membered ring
“locked” the bowl configuration, preventing a rapid bowl-
to-bowl inversion process.21 However, no such effect was
observed for 23b with the two sets of the methylene
hydrogens appeared as two sharp singlets at δ 4.41 and 3.29
ppm and the two groups of methyl hydrogens appeared as a
sharp singlet at δ 1.63 ppm at 25 °C. At -33 °C, the
methylene signals, recorded on a 600-MHz NMR spectrom-
eter, became two broad humps ranging from ca. δ 4.6 to 4.2
ppm and from ca. δ 3.4 to 3.1 ppm, and the methyl signal
also appeared as a broad peak ranging from ca. δ 1.8 to 1.5
ppm. Further broadening but without the appearance of
distinct signals for the diastereotopic hydrogens was observed
at -40 °C, suggesting relatively fast bowl-to-bowl inversion
on the NMR time scale even at -40 °C.
A new synthetic route to buckybowls was developed. The
convergent assembly of the benzannulated enediynyl prop-
argylic alcohols and the efficient cascade cyclizations of the
resulting enyne-allenes leading to polycyclic aromatic di-
bromides for subsequent palladium-catalyzed intramole-
cular arylation reactions are the key features of this syn-
thetic pathway. The use of benzannulated enediynyl prop-
argylic alcohols to form the corresponding aromatic dibro-
mides represents a new approach to the preparation of
buckybowl precursors. Three new buckybowls with their
structures confirmed by X-ray structure analyses were thus
synthesized.
Figure 3. X-ray crystal structure of 23b.
membered ring in 23b appears to cause its structure to be
more strained than those of 16 and 23a. As a result, the
transformation from 22b to 23b is less efficient.
Acknowledgment. The financial support of the Petro-
leum Research Fund (38169-AC1), administered by the
American Chemical Society, and the National Science
Foundation (CHE-0414063) is gratefully acknowledged.
K.K.W. thanks the National Science Council of Taiwan, the
Republic of China, for a grant to support sabbatical leave in
the Department of Chemistry at the National Taiwan
University. J.L.P. acknowledges the support (CHE-9120098)
provided by the National Science Foundation for the acquisi-
tion of a Siemens P4 X-ray diffractometer. The financial
support of the NSF-EPSCoR (1002165R) for the purchase
of a 600-MHz NMR spectrometer is also gratefully acknowl-
edged.
Based on the geometry obtained from the X-ray analysis,
one of the central carbon atoms of 23b has a POAV1 angle
of 10.3° (Figure 4), which is larger than those of 2 and
Supporting Information Available: Experimental pro-
cedures, spectroscopic data, and 1H and/or 13C NMR spectra
of 7, 8, 10, 11, 15-17, and 20a,b-24a,b; ORTEP drawings
for 16, 23a, and 23b; and X-ray crystallographic data of 16,
23a, and 23b (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
OL0605676
Figure 4. POAV1 pyramidalization angles of 23a and 23b.
(21) Abdourazak, A. H.; Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc.
1993, 115, 3010-3011.
corresponds to 88% of that found for C60 with all carbon
2316
Org. Lett., Vol. 8, No. 11, 2006